A conventional magnetic head-positioning device using a voice coil has a disadvantage in that the control circuit for controlling the voice coil motor is very expensive. A voice coil motor generates a driving force operating on the same principles as a dynamic loudspeaker. That is, the presence of a current within a conductive material in a magnetic field induces a driving force in the conductive material. In general, in a conventional magnetic head-positioning device using a voice coil motor, the magnetic head detects a signal which is stored on the servo track giving information of position so that it is possible to determine the position. For transfer between one track and an adjacent track, the magnetic head is controlled by changing the control mode in the section to the desired track position. That is, a position sensor using a servo disk and a magnetic head detects only the track position. Next, a counter counts the number of tracks which the magnetic head passes on the way to the desired track position. The magnetic head is accelerated or slowed down in speed in accordance with the counted value.
Further, a pulse motor has also been used as a magnetic head-positioning device for a magnetic disc drive. However, a magnetic head-positioning device having a pulse motor also has the disadvantage of large size. For example, with reference to a minifloppy disc drive SA 400 as manufactured by the Shugart Company in the United States, the magnetic head is positioned by converting the turning motion of a step motor, having a diameter of approximately 55 mm and a height of approximately 25 mm, to a rectilinear force by means of a cam. Frequently, in the art, the magnetic head-positioning device for a mini-floppy disc drive uses a step motor having the size described above. The shape and size of a magnetic head-positioning device depends on the size of the step motor. Therefore, it is difficult to manufacture a magnetic head-positioning device with a step motor, and including a cam which is less than 30 mm in one dimension. Furthermore, precision of positioning the head is limited to .+-.20 microns. As a result, a magnetic head-positioning device with a step motor is disadvantageous for a small mini-floppy disc device and also for a rigid disc device with regard to cost.
What is needed is a magnetic head-positioning device for a magnetic disc drive which is low in cost, small in size and accurate in positioning the magnetic head relative to the disc.